Why is my car still overheating and why does the car squeal when using the AC?

Replaced engine is the same that was originally in it. The car is a 1996 Mitsubishi Galant. The engine specs are the same as the old motor. 1996-97, 2.4L, 4 cylinder. My VIN#: 4A3AJ46G5TE402615 (on title). VIN# on receipt 4A3AJ56G5VE052718. The 8th digit matches. I got the engine from a reputable salvage yard and engine came with a 30 day warranty which now has expired. The problem has been the same since it was installed and a mechanic installed the replacement engine. HELP!

Oh, by the way, forgot to mention that I need to replace part of the exhaust from the manifold to the catalytic converter. Need to also replace the front o2 sensor. Could that be the problem? Still squeals by the way.

I have to replace the exhaust because the original one had a hole in it and a temporary fix was flex pipe to quiet down the car so that it was driveable but inorder for the flex pipe to fit had to cut off the O2 sensor. The cat isn't plugged to my knowledge. The car has two fans and they both come on when the car reaches the proper temperature. Did check the belts and they are all fine and show no apparent wear to indicate a problem. I'm a single mom and really don't have extra money for extensive or expensive repairs. I thought about the squealing issue. Could the problem be a bad tension pulley?